After-action for :assign_*-style update actions on Diffo Instance resources
that declare pools.
Applies the :assignment argument via Assigner.assign/3 against the named
pool, then reloads the instance through its primary read so preparations
re-run.
Pass the pool name as an option:
update :assign_pair do
argument :assignment, :struct, constraints: [instance_of: Assignment]
change {DiffoExample.Changes.Assign, pool: :pairs}
end